8
Pulsed-Laser Ablation of Au Foil
in Primary Alcohols Influenced
by Direct Current
Karolína Šišková
Dept. of Physical Chemistry, RCPTM,
Palacky University in Olomouc
Czech Republic
1. Introduction
Almost two decades ago, Henglein pioneered the application of laser pulses for the
synthesis of nanoparticles (Amendola & Meneghetti, 2009, as cited in Henglein et al., 1993).
Since that the pulsed-laser ablation process of a foil performed in liquids is one of the top-
down processes of nanomaterials generation. In a nutshell, laser pulses are focused into a
metallic target immersed in a particular liquid producing thus nanoparticles dispersions
(Amendola & Meneghetti, 2009; Georgiou & Koubenakis 2003; Zhigilei & Garrison, 1999).
Noble metal nanoparticles are frequently formed by this approach because of a well-known
fact that the as-prepared nanoparticle solutions do not contain any by-products and
chemicals remaining from usual bottom-up approaches such as chemical syntheses. Hence,
pulsed-laser ablation constitutes a “green” technique of nanoparticles formation.
There are several other benefits which make pulsed-laser ablation process attractive. One of
them lies in the choice of ablation medium which is usually determined by a further usage
of noble metal nanoparticles. So far, numerous papers have been written about pulsed-laser
ablation performed in water and in aqueous solutions of simple ions (e.g. Procházka et al,
1997; Srnová et al, 1998; Šišková et al, 2008), surfactants (e.g. Fong et al, 2010), organic
molecules (e.g. Darroudi et al, 2011; Kabashin et al, 2003; Mafune et al, 2002; Šišková et al,
2007, 2008, 2011), or even DNA (Takeda et al, 2005). In the literature, there have also been
published pulsed-laser ablation processes of metallic foils performed in ionic liquids
(Wender et al., 2011), or in a wide range of organic solvents, such as chloroform
(Compagninni et al., 2002; Mortier et al, 2003; Šišková et al, 2010), toluene (Amendola et al.,
2005), tetrahydrofurane (Amendola et al., 2007), dimethylsulfoxide (Amendola et al., 2007),
N,N-dimethylformamid (Amendola et al., 2007), acetonitrile (Amendola et al., 2007), acetone

unique optical, electrical, and magnetic properties which differ from bulk materials
(Roduner, 2006). In particular, it has been demonstrated that noble metal nanoparticles (Ag,
Au, Cu) possess surface plasmons which are responsible for enhanced light scattering and
absorption (Le Ru & Etchegoin, 2008). This characteristic property of noble metal
nanoparticles is fully exploited in surface-enhanced Raman scattering (SERS) spectroscopy.
Recently, noble metal nanoparticles have also been employed in cancer diagnosis and
therapy (Jain et al., 2007) as well as in photovoltaic devices (Atwater, H.A. & Polman A.,
2010; Kim et al., 2008; Morfa et al., 2008; Tong et al. 2008).
According to a particular exploitation, either liquid dispersions of nanoparticles, or
nanoparticles deposited on a substrate are preferentially required. Noble metal
nanoparticles can be deposited on a particular substrate by several different ways
depending on the force which is responsible for nanoparticles assembling. Roughly divided,
nanoparticles assembling can be directed by molecular interactions, or by external fields as
reviewed in more details in (Grzelczak et al., 2010). An elegant method is to allow self-
assembling of nanoparticles exploiting spontaneous processes (Rabani et al., 2003; Siskova et
al., 2011).
When molecular interactions are intended to be exploited for nanoparticles assembling,
either substrate or nanoparticles have to be suitably modified by a surface modifier which
enables the mutual interaction between nanoparticles and substrates. As an excellent
example, the modification by amino- and/or mercapto-alkylsiloxane, or porphyrins can be
referenced (Buining et al., 1997; Doron et al., 1995; Grabar et al., 1996; Šloufová-Srnová &
Vlčková, 2002; Sládková et al., 2006). Obviously, surface modifications may be useful in or,
on the contrary, disable some applications because they change electrical and optical
properties of nanoparticles as well as of substrates (Carrara et al., 2004; de Boer et al., 2005;
Durston et al., 1998; Rotello, 2004; Schnippering et al., 2007; Wu et al., 2009). Therefore,
many research groups look for other types of nanoparticles assembling. One of many

Pulsed-Laser Ablation of Au Foil in Primary Alcohols Influenced by Direct Current

153

solutions of Au nanoparticles influenced by direct current, but also microscopic and
spectroscopic characteristics of three selected types of substrates which Au nanoparticles are
deposited on due to electrophoresis.
Last, but not least, a possible elucidation of the influence of direct current value on the
mechanism of Au nanoparticles generation by the pulsed-laser ablation process combined
with electrophoretic deposition and performed in primary alcohols is suggested.
2. Experimental
2.1 Materials
Ethanol and butanol of UV-spectroscopy grade purchased from Fluka were used. Cleaning
of a pure Au foil (99.99%, Aldrich) and ablation cell by washing in piranha solution
(H
2
O
2
:H
2
SO
4
, 1:1) was carried out. The latter was also washed with aqua regia
(HNO
3
:HCl, 1:3) in order to remove any residual Au nanoparticles from the previous
experiments. Indium-tin-oxide (ITO) and fluorine-tin-oxide (FTO) coated glass substrates
purchased from Aldrich were ultrasonicated in acetone (p.a., Penta) and dried by nitrogen
flow prior to their use as electrodes in the course of the simultaneous pulsed-laser

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ablation and electrophoretic deposition process. Alternatively, freshly cleaved highly


155
double-beam spectrophotometer (Perkin-Elmer Lambda 950). Zeta-potentials were
measured by means of Zetasizer Nano series (Malvern Instruments). Transmission electron
microscopy (TEM) was used for the characterization of sizes of Au nanoparticles dispersed
in alcoholic solutions after the simultaneous pulsed-laser ablation and electrophoresis. TEM
imaging of dried drops of the Au nanoparticle solutions deposited on a carbon-coated Cu
grid was performed using a JEOL-JEM200CX microscope. Scanning electron microscopy
(SEM) was employed for the characterization of ITO- and/or FTO-coated glass substrates.
SEM images were recorded on a SEM microscope Quanta 200 FEI. HOPG substrates were
measured on Ntegra scanning tunnelling microscope (STM). Mechanically clipped Pt/Ir tip
was approached toward a sample until a set tunnel current was detected. All STM
experiments were done under ambient conditions. STM images were recorded and treated
by using Nova 1.0.26 software provided by NT-MDT.
3. Results and discussion
Our choice of Au target, primary alcohols, and the other parameters for the combined
pulsed-laser ablation and electrophoretic deposition (PLA+EPD) process has been
influenced by several good reasons. First of all, Au nanoparticles are preferred by many
applications as it has been well documented in Introduction. Furthermore, they do not
undergo surface oxidation as easily as Ag and/or Cu nanoparticles (Muto et al., 2007).
Primary alcohols as ablation medium have been chosen because of a good stability of Au
nanoparticles in ethanol and other aliphatic alcohols as reported in the literature many times
(Amendola et al., 2006, 2007; Amendola & Meneghetti, 2009; Compagnini et al., 2002, 2003).
Laser pulses of nanosecond time duration have been rather used because the occurrence of
explosive boiling or other photomechanical ablation mechanisms is suppressed in
comparison to the situation when using femtosecond pulses (Amendola & Meneghetti,
2009). The 532 nm wavelength has been employed in our study owing to the fact that a
narrow particle size distribution can be obtained due to an efficient Au nanoparticles
fragmentation accompanying their generation (by pulsed-laser ablation) at this wavelength.
The selection of substrate types serving as electrodes is given by possible applications of Au

Fig. 1. UV-vis extinction spectra of Au nanoparticles generated by PLA+EPD process in
ethanol while direct current of 10 μA and/or 17 μA passed through.
The maximum of surface plasmon extinction of Au10 is located at 522 nm, while that of
Au17 is placed at 517 nm - Figure 1. Considering that all the other conditions, except for the
direct current value, are the same (duration of PLA+EPD, laser fluence, experimental setup,
etc.), and taking into account Mie theory (Rotello, 2004), the average nanoparticle size of
Au17 could be smaller than that of Au10. This assumption is corroborated by particle size
distribution (PSD) determined on the basis of TEM imaging – Figure 2. While Au10 contains
the nanoparticles of 7.3 ± 3.1 nm in diameter (Figures 2A,B), nanoparticles of 4.0 ± 0.9 nm in
diameter are encountered in Au17 (Figures 2C,D).
Interestingly, the optical density of Au10 is slightly higher than that obtained for Au17
(Figure 1) which can be related to a lower concentration of nanoparticles in Au17 solution.
The decrease of Au nanoparticles concentration in Au17 solution is most probably caused by
a higher amount of electro-deposited Au nanoparticles on electrode surface when the direct
current of 17 μA is passed through the ablation medium. This hypothesis will be discussed

potentials of these systems are presented in Table 1. They indicate rather unstable Au
nanoparticles solutions since the values are above -30 mV and below 30 mV. The differences
in zeta potential values of Au10, Au17, and Au10B, Au17B can be ascribed to different

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dielectric constants of solvents: ethanol possess the value of 24.3, while butanol 17.1
(Sýkora, 1976).
UV-visible extinction spectra of Au10B and Au17B solutions are shown in Figure 3. Both
systems manifest themselves by a well pronounced surface plasmon extinction band with
the maximum located at 526 nm indicating thus similar sizes of Au nanoparticles. This idea
has been confirmed by PSD based on TEM imaging, presented in Figure 4. Au nanoparticles
in Au10B solution reveal sizes of 4.9 ± 1.2 nm and in Au17B sizes of 5.2 ± 1.7 nm
in diameter.

Pulsed-Laser Ablation of Au Foil in Primary Alcohols Influenced by Direct Current

159

Fig. 4. TEM images (A, C) and appropriate PSD (B, D) of Au nanoparticles formed by
PLA+EPD process while 10 μA (A, B) and/or 17 μA (C, D) passing through butanolic
ablation medium.
3.2 Substrates with electrophoretically-deposited Au nanoparticles
In this section, three types of substrates serving as electrodes in the PLA+EPD process will
be characterized by means of microscopic techniques and visible absorption spectroscopy.
With respect to the negative values of zeta potential of generated Au nanoparticles in both
primary alcohols, they are preferentially deposited on anodes.
3.2.1 ITO-coated glass substrates
SEM images of the ITO-coated glass substrates modified by electrodeposited Au
nanoparticles during the PLA+EPD process performed in ethanol are shown in Figure 5.
Comparing the SEM images of substrates in Figure 5A (10 μA direct current) and 5B (17 μA
direct current), a higher surface coverage of substrates by Au nanoparticles is observed at
higher current values than at the lower one. This microscopic observation goes hand in hand
with the fact deduced from the UV-visible extinction spectra of Au nanoparticles solutions
(discussed in the previous section): the final concentration of Au17 solution is lower than
that of Au10 solution. The reason for this difference lies in a larger amount of Au
nanoparticles being deposited under the higher than the lower current value and, as a
consequence, a decrease of Au nanoparticles concentration in Au17 solution being
determined.

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Fig. 5. SEM images (A, B) and particular differential visible extinction spectra (C, D) of ITO-

values employed. The positions of the maxima of surface plasmon extinction bands correlate
with the microscopic observation presented in Figures 6A and 6B. Indeed, the higher the
surface coverage of substrates by Au nanoparticles, the more intense and red-shifted surface
plasmon extinction observed. In comparison to the extinction spectra of substrates
immersed in ethanolic solutions during the PLA+EPD process, the surface plasmon
extinction band is well-developed at both current values exploited for the PLA+EPD process
performed in butanol. Thus, regarding the aggregation of Au nanoparticles electro-
deposited on ITO-coated glass substrates, it is less pronounced in butanolic than in ethanolic
samples. Again, the same result evidenced by two independent methods, microscopic and
spectroscopic one. Fig. 6. SEM images (A, B) and particular differential visible extinction spectra (C, D) of ITO-
coated glass substrates modified by Au nanoparticles electro-deposited at 10 μA (A, C)
and/or 17 μA (B, D) during PLA+EPD process performed in butanol.

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3.2.2 FTO-coated glass substrates
As it has been already evidenced in the previous section, there are strong effects of direct
current value and the type of alcohol on the final coverage of a substrate by Au
nanoparticles. In order to investigate if there is any additional influence of substrate
roughness, FTO-coated glass substrates have been used as electrodes during the PLA+EPD
process performed in ethanol at both values of direct current, 10 μA as well as 17 μA.
In Figure 7A, the SEM image of a cleaned bare FTO-coated glass substrate surface is shown.
Obviously, the surface of a FTO-coated glass substrate is very rough with plates and crystals
being of sizes of hundreds of nanometers. Taking into account that the local current density
can be very different on the edges of a plate and/or crystal, an inhomogeneous distribution
of Au nanoparticles and their aggregates on FTO-coated glass substrate can be awaited.

With respect to the fact that Au nanoparticles in the selected system are tiny (around 4 nm
in diameter), another microscopic technique than SEM has to be employed in order to
visualize isolated nanoparticles on HOPG substrates. Scanning electron microscopy (STM)
can fulfil this task when appropriate measuring conditions met (Durston et al., 1998; Wang
et al., 2000).
Figure 8 shows topographic as well as tunnelling current images of a freshly cleaved HOPG
surface without and with electrodeposited Au nanoparticles. Smoothness of HOPG surface
is well evidenced in Figure 8A where the value along z axis (perpendicular to the plane of
the image) stays well below 1 nm. The values of tunnelling current below 0.2 pA have been
recorded on a freshly cleaved HOPG substrate measured under ambient conditions, in air
and at the room temperature – Figure 8B. Under the same conditions, STM measurements of
a HOPG substrate which served as the anode during the PLA+EPD process have been
undertaken and one of the resulting topographic images together with its tunnelling current
values are shown in Figures 8C and 8D, respectively. Evidently, isolated Au nanoparticles
are randomly, however, quite homogeneously dispersed on the surface of a HOPG plate
(Figure 8C), the value of 6 nm along z axis is not surpassed. It is worth noting that
tunnelling current exceeds 0.4 nA (Figure 8D), which is the value of more than three orders
of magnitude higher than on a bare HOPG substrate (Figure 8B). This can be related to the
presence of Au nanoparticles.
It is known that HOPG substrate can contain terraces and steps as observed in Figure 8C.
Hypothetically, the edges of these terraces and steps could be the places of a locally higher
electrical density, hence, more electrodeposited Au nanoparticles could be awaited to occur
on these edges. However, this was not the case as evidenced in Figure 8C.
Figure 9 shows topography and tunnelling current image of a smaller flat surface area (200 x
200 nm
2
) on a HOPG substrate decorated with electrodeposited Au nanoparticles. At this
place it should be noticed that the bias voltage of +0.1 V has been applied between the
measured HOPG substrate and the tip during STM imaging. This is a sufficiently low value
to suppress any unwanted manipulation of Au nanoparticles (Durston et al., 1998).

nanoparticles generation during PLA+EPD process in primary alcohols
Since the final stages of Au nanoparticles solutions and/or the electrodeposited Au
nanoparticles on different substrates have been investigated, it cannot be unambiguously
stated the exact formation mechanism of Au nanoparticles during the PLA+EPD process.
However, it can be hypothesized the influence of direct current value on the mechanism of
Au nanoparticles generation by the PLA+EPD process in comparison to a generally adopted
mechanism of pulsed-laser ablation itself.
The prevailing formation mechanism of nanoparticles by a classical pulsed-laser ablation
process implies the generation of a plasma plume followed by its cooling (Amendola &
Meneghetti, 2009; Tsuji et al., 2004). The former step is nothing else than the vaporization of
the part of a target which was attacked by the focused beam of laser pulses. During the
second step (plasma plume cooling), the formation of nanoparticles nuclei starts. The
driving force for the nucleation is the supersaturation in the plasma plume (Amendola &.
Meneghetti, 2009). Subsequently, the nuclei grow and coalesce into the sizes of resulting
nanoparticles. This last step strongly depends on the polarity of solvents, the presence
and/or the absence of simple ions or adsorbing species which may stabilize nanoparticles of
a particular size.
Under the assumption that our pulsed-laser ablation process in the selected solvent (e.g.
ethanol or butanol) is repeatedly performed in the same way and under otherwise the same
experimental conditions, the value of the applied electric field can induce changes rather in
the step of nuclei growth and coalescence than during the plasma plume generation and/or
the nucleation process. As it has been pointed out in section 3.1, ethanol possesses a higher
dielectric constant than butanol which means that ethanol is more easily polarized by an
increasing electric field than butanol. This implies that a further nuclei growth and
coalescence is possibly hindered in the case of 17 μA direct current value passing through
the ethanolic ablation medium when pulsed-laser ablation takes place. Basically, charged
nanoparticles of smaller sizes in diameter can be efficiently stabilized in a more polarized
solvent, i.e., in our case, at a higher current value passing through the ethanolic ablation
medium. Thus, a smaller average particle size is observed in Au17 than in Au10 solutions.
Nevertheless, this hypothesis needs a further experimental support which is beyond the

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